What EU member states should do after Brexit

Push for more empowered EU with more effectiveness, expanding the vote by majority for further unification

Negotiate ruthless economic terms with UK after exit in order to discourage other member states that may want to do the same (it is very concerning that UK stocks lost 2% while Paris and Milan lost 8 and 12%

Assume other nations will leave in the next years, and that will be just fine as it will make life easier for those that remain.